The present invention relates in general to diode-laser light-sources. It relates in general to a light source for projecting light from a linear diode-laser array into an elongated line of light.
Diode-lasers are commonly used as sources of illumination in various graphics applications such as display systems, optical printing systems and optical recording systems. In one type of prior-art imaging system a linear (one-dimensional) array of light modulators is illuminated by an illuminator including a light source. Illumination from the array of modulators is projected onto a recording medium or the like in the form of a line of images of the modulators. By scanning the recording medium past the line of images and appropriately synchronizing the scanning with operation of the modulator array, the modulator array is used to draw a two-dimensional image on the recording medium.
A preferred light-source for the illuminator is a linear array of diode-lasers commonly referred to as a diode-laser bar. A diode-laser bar can efficiently provide relatively high illumination power, for example 60.0 Watts (W) or more, from a source having a maximum overall dimension no greater than about 10 centimeters (cm). A disadvantage, however, is that each of the diode-lasers in the bar is an individual emitter. This presents problems in selecting an appropriate optical configuration for the illuminator.
This problem is addressed in one prior-art illuminator by including a diode-laser array (or correspondingly an array of light-emitting diodes) having a number of emitters equal to the amount of modulators to be illuminated, and an optical system configured to image each emitter onto a corresponding modulator. This type of illuminator has a disadvantage that it is dependent on all emitters continuing to function. Failure of one emitter could cause at least a reduction in performance of the recording system, for example, a black line on an image in the direction of scanning.
An illuminator arrangement for overcoming this disadvantage is disclosed in U.S. Pat. No. 5,517,359 (Gelbart et al.). Here an optical system is used to project elongated overlapping images of each individual emitter onto a modulator array. The number of emitters need not correspond to the number of modulators, and failure of an individual emitter is described as merely reducing the overall illumination on the modulator rather than effecting the spatial distribution of illumination on the modulator.
An important feature of the Gelbart et al. system is an array of microlenses which are used in forming the elongated images. The microlenses correspond in number to the number of emitters in the diode-laser bar, but are spaced closer together than the emitters in a predetermined ratio of emitter and microlens spacing. It is believed that the requirement for such a microlens array imposes certain cost and manufacturing disadvantages on the illuminator. By way of example, normal manufacturing variations in spacing of both emitters and microlenses could lead to a requirement for matching individual diode-laser bars to individual microlens arrays. Further, even given an appropriately matched diode-laser bar and microlens array, alignment of one with the other may be very difficult. This is because the microlens array must be aligned laterally with respect to the emitters such that the lateral spacing between end ones of the microlenses and arrays is equal. This requirement of course is in addition to a requirement that the microlens array be a ligned, parallel to the line of emitters and at the correct distance from the emitters.
There is a need for a diode-laser bar powered illuminator which projects a line of light wherein any point is illuminated about equally by all emitters of the diode laser bar, but which does not require an optical system including an array of microlenses to accomplish this result.
In one aspect of the present invention, apparatus for projecting a line of light includes a plurality of diode-lasers spaced apart in an elongated linear array. The linear array has a slow axis which is parallel to the length direction of the diode-laser array, and a fast axis perpendicular to the slow axis. Light is emitted from each of the diode-lasers as diverging rays in both the fast and slow axes. The rays are emitted in a general direction of propagation mutually perpendicular to both the fast an slow axes. An optical system is arranged with a longitudinal axis thereof parallel to the propagation direction of light from the diode-lasers. The optical system is arranged to focus the fast-axis diverging rays in a focal plane perpendicular to the longitudinal optical axis, and arranged to form the slow-axis diverging rays into a plurality of bundles of parallel rays, one thereof for each diode-laser. The bundles of parallel rays intersect in the focal plane. The optical system thereby causes light from the diode-lasers to be formed into a line of light in the focal plane. The line of light has a width in the fast axis and a length in the slow axis.
In another aspect of the present invention, the emitting apertures of the diode-laser bars are located in a slow-axis object plane of the optical system and the bundles of parallel rays intersect in a slow-axis exit-pupil of the optical system. Fast axis rays are focussed in the slow-axis exit-pupil thereby providing the line of light. The line of light has a width equal to about the fast-axis focal spot-size and a length about equal to the width of the slow-axis exit-pupil.
In yet another aspect of the present invention, the optical system does not include any microlens arrays wherein individual optical elements operate on rays from corresponding individual diode-lasers. In the inventive optical system, rays from all diode-lasers traverse all optical elements of the optical system.
In still another aspect of the present invention the optical system may include one or more physical stops arranged to prevent selected portions the fast and slow-axis diverging rays from the diode-laser array from reaching the fast-axis focal plane. The selected portions are chosen to optimize the intensity of light distribution along the length of the line of light. In one preferred embodiment, each of the physical stops is an elongated stop formed from a length of optical fiber having a length extending completely through the fast-axis rays from the diode-laser array.